This invention relates to a method and apparatus for surface polishing of flat workpieces in the form of films or sheets; such as the surface of interlayer films deposited in the course of device fabrication, e.g., of memories and logic devices; glass substrates of liquid crystal filters; and substrates of plasma display units.
A flat sheet-like workpiece W of the character, for example, a wafer on which devices are to be fabricated, takes the form of a disk as illustrated in FIG. 12. Typically, on a silicon wafer a, as shown in FIG. 13, a first layer of conductive pattern b of aluminum or other conductive metal is formed; an interlayer film c of a highly dielectric metal of SiO.sub.2 system or the like is deposited on the conductive pattern b as in FIG. 14; the surface of the interlayer film c is smoothed by polishing and formed with contact holes d as in FIG. 15; a second patterned conductive layer e is formed as in FIG. 16; and the foregoing series of steps is repeated to form an interlayer film on the second patterned conductive layer e and polish and smooth the surface of the interlayer dielectric film c; and ensuing layers are deposited one over another to form a 4-, 5-, 6-, or even more multiple layer laminate of a highly complex patterned conductive structure.
As apparatus for polishing the surface of such flat sheetlike workpieces, lapping machines are in common use. To prevent slight polishing strains and scratches, they use a pad of preselected hardness or a lapping table or pad having surface irregularities in a predetermined pattern. Abrasive in the form of free grains is supplied to the space between the lapping table or pad and the workpiece, and the latter surface is polished while the opposing surfaces and grains in between are all in a rotating mode.
With the conventional arrangement, for example, for the polishing of a device wafer surface, it is thought necessary to remove dents and other surface projections preferentially and remove all other irregularities to produce a uniformly smooth surface, irrespective of the dimensions of the device pattern or of the density of the substrate texture. These requirements combine with slender machining allowances to make the operating conditions of the lapping machine most exacting. Moreover, the recent tendency toward larger device wafers has called for more massive lapping machines with greater complexities in handling the wafers, sometimes at a sacrifice of the operating efficiency. Further, where the construction is such that the lapping table or pad has patterned surface irregularities, the lapping machine is very expensive to manufacture and yet is not assured of durability. Such machines score low on both economy and high speed machinability.